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A long way down…. How would you describe a high diver? As someone who is:. brave. insane. full of beans. full of gravitational potential energy (GPE). GPE is the amount of energy an object has because of its position above the ground, i.e. its height.
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A long way down… How would you describe a high diver? As someone who is: • brave • insane • full of beans • full of gravitational potential energy (GPE). GPE is the amount of energy an object has because of its position above the ground, i.e. its height.
What is gravitational potential energy? The gravitational potential energy (GPE) of an object on Earth depends on its mass and its height above the Earth’s surface. • When a bungee jumper starts to fall he starts to lose GPE. • As the elastic cord pulls the bungee jumper back up, he gains GPE.
GPE = mass x gravitational field strength x height How is GPE calculated? The GPE of an object can be calculated using this equation: • Mass is measured in kilograms (kg). • Gravitational field strength is measured in newtons per kilogram (N/kg), usually taken as 10N/kg on Earth. • Height is measured in metres (m). • GPE is measured in joules (j).
Calculating GPE question 1 An osprey with a mass of2kg flies at a height of200m above the ground. How much gravitational potential energy does the osprey have? GPE = mass x gravitational field strength x height = 2 x 10 x 200 = 4,000J
changein height GPElost = mass x gravitational field strength x Calculating GPE question 2 An apple with a mass of 200g falls 3m from its branch to the ground. How much GPE will the apple have lost when it reaches the ground? = 0.2 x 10 x 3 = 6J
Feeling energetic? Who has the most kinetic energy: Rita or the cat?
What is kinetic energy? The word ‘kinetic’ comes from the Greek word ‘kinesis’, meaning motion. Kinetic energy is the energy an object has because it is moving. All moving things have kinetic energy, but the amount of energy they have is not just dependent on how fast they are moving. What other factors affect the kinetic energy of a moving object?
KE = ½ x mass x velocity2 = ½mv2 How is kinetic energy calculated? The kinetic energy (KE) of an object can be calculated using this equation: • Mass is measured in kilograms (kg). • Velocity is measured in metres per second (m/s). • KE is measured in joules (j).
Calculating kinetic energy question A car with a mass of 1,500kg travels at a velocity of 20m/s. What is the kinetic energy of the car? kinetic energy = ½ xmass x velocity2 = ½ x 1,500 x 202 = 300,000J = 300kJ
KE = ½mv2 Ö 2KE m 2KE v2 v = m = Rearranging the KE equation Sometimes it is necessary to rearrange the kinetic energy equation in order to calculate the mass or the velocity of a moving object. What are the rearranged versions of this equation for calculating mass and velocity?
Ö 2KE mass velocity = Ö 2 x 2,250,000 20,000 = Calculating velocity question A lorry has a mass of 20,000kg. If its kinetic energy is 2.25MJ, at what velocity is it travelling? KE = ½ xmass x velocity2 = 15m/s
1,000kg 2,000kg 20mph 40mph Dangerous speeding? Use the KE = ½mv2 equation to fill in the kinetic energy values in the table below for two cars each travelling at two different velocities. KE = 40kJ KE = 80kJ KE = 160kJ KE = 320kJ What factor – mass or velocity – has the greatest effect on the kinetic energy of a moving object?
Too much kinetic energy Doubling the mass of a moving object doubles its kinetic energy, but doubling the velocity quadruples its kinetic energy. If the velocity of a car is slightly above the speed limit, its kinetic energy is much greater than it would be at the speed limit. This means that: • It is more difficult to stop the car and there is more chance of an accident. • It the car does collide with something, more energy will be transferred, causing more damage.
Energy cannot be created or destroyed, just changed in form. What is conservation of energy? There are many different forms of energy, such as kinetic, sound, thermal and light energy. Each form of energy can be transferred or converted into an another form. All energy transfers follow the law of conservation of energy: This means that energy never just ‘disappears’. The total amount of energy always stays the same, i.e. total input energy = total output energy. In most energy transfers, the energy is transferred to several different forms, which may or may not be useful.
Energy transfer of falling objects What happens to the KE and GPE of a rollercoaster?
GPE lost = KE gained The relationship between GPE and KE The law of conservation of energy means that as an object falls, the GPE it loses must turn into a different form. This is only true if air resistance and friction are ignored. In reality, GPE would also be transferred into heat and sound energy so the KE of rollercoaster would be less than the GPE lost.
Glossary • conservation of energy – The law that states that energy cannot be created or destroyed, just transferred into different forms. • energy transfer – A process in which one form of energy is transferred into another form. • gravitational field strength –The strength of gravity in a particular location. • gravitational potential –The energy an object has because of its position in a gravitational field. • joule – The unit of energy. • kinetic –The energy an object has because it is moving. • velocity – A measure of the speed and direction of a moving object.
